1. Field of the Invention
The present invention relates to a television signal transmitter used in for example a CATV system, for frequency converting intermediate frequency band signals of television to the television signals of broadcasting channels.
2. Description of the Related Art
FIG. 2 shows a circuit arrangement of primary part of a television signal transmitter in the Prior Art, having two systems of frequency converters, namely a first frequency converter 11 and a second frequency converter 12 for frequency converting intermediate frequency band signals of television (referred to as IF signal herein below) to television signals of broadcasting channel frequency (referred to as television signals, herein below) . These frequency converters 11 and 12 are housed in a same housing.
The first frequency converter 11 and the second frequency converter 12 have the identical configuration; the first frequency converter 11 has a first mixer 11a for frequency converting the IF signals to a first IF signals of higher frequency, a first local oscillator 11b for inputting to the first mixer 11a first local oscillation signals, a second mixer 11c for frequency converting the first IF signals to the television signals, and second local oscillator 11d for inputting to the second mixer 11c second local oscillation signals.
Similarly, the second frequency converter 12 has also a first mixer 12a, first local oscillator 12b, second mixer 12c, and second local oscillator 12d. 
The contents of IF signals input to the first mixers 11a and 12a of respective first and second frequency converters 11 and 12 (program contents) are different from each other. Thus the television signals output from the second mixers 11c and 12c are output at mutually different channel frequency. These television signals are mixed by a mixer 13 to send to subscribers of a CATV system through cables not shown in the figure.
In the above configuration, the frequency of first IF signals is preset to a predefined frequency so that the same frequency of first local oscillation signals will be output from two first local oscillators 11b and 12b respectively. Since the channel of television signals output from the second mixers 11c and 12c are mutually different from each other, the frequency of second local oscillation signals output from each of second local oscillators 11d and 12d will be different from each other by frequency of at least one channel.
In the television signal transmitter of the Prior Art as above, the first local oscillation signals are preset to the same frequency, however, in practice, these frequencies are not perfectly matched but slightly different from each other due to the constant inconsistency of parts used for the first local oscillators.
Therefore, for example, it is possible that the first local oscillation signals of the first frequency converter may be leaked and input to the second frequency converter, or on the contrary, the second local oscillation signals of the second frequency converter is leaked and input to the first frequency converter. Then, in each frequency converter, first local oscillation signals and second local oscillation signals may be intermixed at the first mixer or elsewhere to create differential signals of these frequencies. The differential signal level will be larger if the difference in frequencies is smaller, in other words if the frequencies of the first local oscillation signals are closer to each other, thereby to appear as interference in the proximity of video carrier of television signals.
As a result, interference fringes will be appeared on the screen when receiving such television signals, causing considerable degradation of video image quality.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a television signal transmitter that can eliminate interference caused by the differential signal of frequencies generated by the respective first local oscillation signals of two frequency converters.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a television signal transmitter, comprising a first frequency converter and a second frequency converter each outputting by frequency converting the intermediate frequency band television signals into mutually different broadcasting channel television signals, each of the frequency converters including a first mixer for frequency converting television signals of the intermediate frequency band to television signals of first intermediate frequency band of frequency higher than the television signals frequency of the intermediate frequency band, a first local oscillator for generating first local oscillation signals to be input to the first mixer, a second mixer for frequency converting the first intermediate frequency band television signals into the broadcasting channel television signals, and a second local oscillator for generating second local oscillation signals to be input to the second mixer; wherein the frequency of the first local oscillation signals in the first frequency converter is different from the frequency of first local oscillation signals in the second frequency converter by 0.25 MHz.
In addition, the television signal transmitter in accordance with the present invention further comprises a band pass filter, between the first mixer and the second mixer, having a broader pass band than the frequency band of the first intermediate frequency band television signals in both the first frequency converter and the second frequency converter, wherein the frequency of first local oscillation signals in the first frequency converter and the frequency of first local oscillation signals in the second frequency converter may be set to any one of frequencies of integer times of 0.25 MHz respectively with respect to a predetermined reference frequency.
In addition, the television signal transmitter in accordance with the present invention further comprises a PLL circuit separately for each of the frequency converter for controlling respectively the first local oscillator and a microcomputer for storing frequency data for setting the frequency of first local oscillation signals in the first frequency converter and the frequency of first local oscillation signals in the second frequency converter, wherein the microcomputer inputs the frequency data to each of the PLL circuits.